This invention relates generally to microwave ovens and more specifically to an improved microwave oven having convection and griddle features.
As is known in the art, microwave ovens have heretofore been provided with a hot bottom, or griddle feature. This arrangement is desirable since the griddle provides browning of the food product not generally realized with microwave cooking. However, while the griddle may adequately brown the bottom surface of the food, top portions may be insufficiently browned.
As is also known in the art, convection cooking capability may be combined with the above microwave and griddle arrangement in order to further enhance browning of the food product. With such an arrangement, heated air is passed over the food product in order to brown top portions thereof.
One example of a microwave oven combining microwave cooking, griddle, and convection features is described in U.S. Pat. No. 3,716,687 entitled "Method and Apparatus for Cooking" issued to R. J. W. Constable. In the unit described, the food product is placed on a grid-like shelf for cooking. Disposed beneath, and spaced from, the shelf is a heated plate or griddle, which radiantly heats food on the shelf. Convection heating is provided by drawing air out of the cooking cavity with a fan through a plurality of apertures disposed in the rear wall thereof. The air is then forced back into the cooking cavity through an aperture disposed peripherally around such rear wall. With this arrangement, some of the air will be heated by the heated plate.
However, the above-described arrangement does not provide optimum cooking uniformity. More particularly, several factors affect the uniformity with which a food product is cooked, such as the temperature uniformity of the surface on which the food is cooked (i.e. the shelf in the above-described arrangement) and the temperature uniformity of the convective air contacting the food product. Moreover, the temperature uniformity of such air is affected by the manner with which the air is heated as well as the air flow path over the food.
In the above-described oven, the shelf temperature is likely to be non-uniform since the radiant heating thereof by the heated plate will vary with the air flow over such plate. In other words, due to the non-uniform flow of recirculated air over the heated plate, the portion of the heated plate adjacent the aperture through which air is forced back into the cooking cavity will tend to be cooler than other portions of the plate. Thus, the portion of the shelf disposed above the cooler portion of the plate will be concomitantly cooler as well.
The temperature of the air contacting the food product is also likely to be non-uniform due to the air flow path back into the cooking cavity. In other words, air entering the cooking cavity at the top portion of the aperture will tend to be cooler than air entering the cavity through the bottom portion of the aperture since the latter will flow closer to the heated plate. Finally, further non-uniformity in the temperature of the air may occur because of the air flow pattern within the cooking cavity. In other words, portions of the food product adjacent the apertures in the cooking cavity rear wall are likely to be cooked faster than portions thereof adjacent to the front wall due to the increased air flow adjacent to the rear wall.
A further disadvantage of the above-described arrangement is the apparent inefficiency of the heat transfer between the heated plate and the convective air. More particularly, because of the way in which the air is forced back into the cooking cavity, only a portion thereof will pass over the heated plate. Thus, maximum heat transfer from the plate to the convective air is not achieved.